It is known to make nonwoven fabrics of polymeric material by, among other steps, separating the polymeric fibers from a bale, either in a dry-laid or wet-laid process, and feeding the fibers into a garnett to be carded, thereby forming a web of nonwoven polymeric fibers. To facilitate formation of the web of polymeric fibers during the carding process, a lubricant may be introduced onto the polymeric fibers. The polymeric fiber web may then be lapped to form multiple layers. During the lapping operation, adjacent layers may be rotated relative to each other by a predetermined angle. The resulting multi-layer polymeric structure can then be needled or needle-punched to interengage fibers of respective layers with each other and thereby form a single fabric of polymeric material. The above-described process steps and the apparatus for accomplishing them are described more fully in "The Non-Woven Fabric Handbook," by The Association of the Non-Woven Fabrics Industry, and in U.S. Pat. No. 4,888,234 to Smith, the teachings of which are incorporated herein by reference.
It is also known to create nonwoven metal fabrics by overlaying portions of a nonwoven, metal web to form a multi-layer structure, and then needling or needle-punching the multi-layer structure to form a coherent metal fabric.
The metal fibers in such structure are formed by shaving a metal member with a serrated blade, the resulting shavings comprising the metal fibers. Although the presence of a lubricant between the metal member and the serrated blade may assist in shaving off metal fibers, a lubricant is not generally used because it remains on the metal fibers of the finished product and interferes with customer acceptance and product function in many applications. As a result, the current art teaches maintaining the metal member and resulting fibers substantially free of any lubricant.
The need to maintain the metal fibers free of lubricant has generally constrained attempts to improve the density, uniformity, strength, heat dissipation, and other characteristics of nonwoven metal fabrics. One attempt at such an improved metal fabric is disclosed in U.S. Pat. No. Re. 28,470 to Webber, but the disclosed porous metal structure and method for making it suffer from additional drawbacks and disadvantages. For example, the process disclosed by Webber for making a metal structure is both far too complex and far too costly for many applications where a nonwoven metal fabric is required. In particular, the metal fibers of Webber are formed by an elaborate process of drawing larger diameter metal wires through various constrictions and by tensioning the resulting fibers until they are less than 50 microns in average diameter. The metal fibers formed by the Webber process have outer surfaces which are not as rough and therefore not as prone to advantageous interengagement as those created by the shaving process discussed above. As a result, Webber requires additional and costly processing steps, such as annealing and compacting, to create a suitably strong, coherent metal structure.
Because the metal fibers resulting from the drawing processes of Webber are smoother than those generated by the shaving process discussed above, and are generally less than 50 microns in average diameter, the fibers of Webber are able to be carded. Unfortunately, however, the Webber process cannot be used for fibers over 50 microns in average diameter, as they generally disintegrate during the process. Thus the Webber process is limited to use with metal fibers under 50 microns in diameter. But such fibers are usually not required by the particular application and, for reasons mentioned above, are too costly for many applications of nonwoven metal fabrics.
As a result, certain textile processing apparatus which might be used to enhance the characteristics of nonwoven metal fabric have not been usable heretofor without disintegration of the constituent metal fibers and a consequent breakdown of any web formed from such fibers.
Accordingly, there is a need for a nonwoven metal fabric which can be economically made using suitably adjusted, current textile processing apparatus.
There is a further need for such metal fabric to have improved uniformity, strength, density, and heat-dissipation characteristics.